Urban Drainage Clarifier Load-Response as a Function of Flow, Unsteadiness, and Baffling

Abstract

A major function of clarifiers (basins or tanks) for urban drainage (stormwater or wastewater) is separation of particulate matter (PM) and PM-associated constituents. Clarifier response is often indexed to geometric parameters of surface area (SA) or length to width (L/W) ratio. Yet in urban areas, clarifier geometry is constrained by infrastructure and alternative opportunity land uses. As a result, retrofitting clarifiers with baffles is considered to improve hydraulic and PM separation response. While most studies evaluate such response based on steady flow, stormwater unit operations (UO) are subject to highly unsteady flows. This study examines baffle configuration [direction and flow tortuosity (Le/L) as an analog for L/W], flow rate, and hydrograph unsteadiness (λ) on hydraulic response through the Morrill index (MI), volumetric efficiency (VE), and N-tanks-in-series (N) metrics. Results are generated from physical and computational fluid dynamics (CFD) models for clarifier designs with and without baffles. For clarifiers without baffles, but of differing cross-sectional geometries, VE and N increased while MI decreased with increasing flow rate. Clarifiers with baffles produced an asymptotic relationship between N and the Péclet number (P). The MI and N trends were a function of λ and Le/L. For a fixed SA, a higher baffle number (higher Le/L) generated greater PM separation. Although there is not a singular hydraulic response for clarifiers as a function of λ; increasing Le/L does confer a more consistent, reproducible response.